If an airplane were viewed in straight and level flight
from the rear (Fig. 17-32), and if the forces acting on the airplane actually
could be seen, two forces (lift and weight) would be apparent, and if the
airplane were in a bank it would be apparent that lift did not act directly
opposite to the weight - it now acts in the direction of the bank. The
fact that when the airplane banks, lift acts inward toward the center of
the turn, as well as upward, is one of the basic truths to remember in
the consideration of turns.

As we learned earlier, an object at rest or moving in a
straight line will remain at rest or continue to move in a straight line
until acted on by some other force. An airplane, like any moving object,
requires a sideward force to make it turn. In a normal turn, this force
is supplied by banking the airplane so that lift is exerted inward as well
as upward. The force of lift during a turn is separated into two components
at right angles to each other. One component which acts vertically and
opposite to the weight (gravity) is called the "vertical component of lift."
The other which acts horizontally toward the center of the turn is called
the "horizontal component of lift." The horizontal component of lift is
the force that pulls the airplane from a straight flightpath to make it
turn. Centrifugal force is the "equal and opposite reaction" of the airplane
to the change in direction and acts equal and opposite to the horizontal
component of lift. This explains why, in a correctly executed turn, the
force that turns the airplane is not supplied by the rudder.

An airplane is not steered like a boat or an automobile;
in order for it to turn, it must be banked. If the airplane is not banked,
there is no force available that will cause it to deviate from a straight
flightpath. Conversely, when an airplane is banked, it will turn, provided
it is not slipping to the inside of the turn. Good directional control
is based on the fact that the airplane will attempt to turn whenever it
is banked. This fact should be borne in mind at all times, particularly
while attempting to hold the airplane in straight and level flight.

Merely banking the airplane into a turn produces no change
in the total amount of lift developed. However, as was pointed out, the
lift during the bank is divided into two components, one vertical and the
other horizontal. This division reduces the amount of lift which is opposing
gravity and actually supporting the airplane's weight; consequently, the
airplane loses altitude unless additional lift is created. This is done
by increasing the angle of attack until the vertical component of lift
is again equal to the weight. Since the vertical component of lift decreases
as the bank angle increases, the angle of attack must be progressively
increased to produce sufficient vertical lift to support the airplane's
weight. The fact that the vertical component of lift must be equal to the
weight to maintain altitude is an important fact to remember when making
constant altitude turns.

At a given airspeed, the rate at which an airplane turns
depends upon the magnitude of the horizontal component of lift. It will
be found that the horizontal component of lift is proportional to the angle
of bank; that is, it increases or decreases respectively as the angle of
bank increases or decreases. It logically follows then, that as the angle
of bank is increased the horizontal component of lift increases, thereby
increasing the rate of turn. Consequently, at any given airspeed the rate
of turn can be controlled by adjusting the angle of bank.

To provide a vertical component of lift sufficient to hold
altitude in a level turn, an increase in the angle of attack is required.
Since the drag of the airfoil is directly proportional to its angle of
attack, induced drag will increase as the lift is increased. This, in turn,
causes a loss of airspeed in proportion to the angle of bank; a small angle
of bank results in a small reduction in airspeed and a large angle of bank
results in a large reduction in airspeed. Additional thrust (power) must
be applied to prevent a reduction in airspeed in level turns; the required
amount of additional thrust is proportional to the angle of bank.

To compensate for added lift which would result if the
airspeed were increased during a turn, the angle of attack must be decreased,
or the angle of bank increased, if a constant altitude were to be maintained.
If the angle of bank were held constant and the angle of attack decreased,
the rate of turn would decrease. Therefore, in order to maintain a constant
rate of turn as the airspeed is increased, the angle of attack must remain
constant and the angle of bank increased.

It must be remembered that an increase in airspeed results
in an increase of the turn radius and that centrifugal force is directly
proportional to the radius of the turn. In a correctly executed turn, the
horizontal component of lift must be exactly equal and opposite to the
centrifugal force. Therefore, as the airspeed is increased in a constant
rate level turn, the radius of the turn increases. This increase in the
radius of turn causes an increase in the centrifugal force, which must
be balanced by an increase in the horizontal component of lift can only
be increased by increasing the angle of bank.

In a slipping turn the airplane is not turning at the rate
appropriate to the bank being used, since the airplane is yawed toward
the outside of the turning flightpath. The airplane is banked too much
for the rate of turn, so the horizontal lift component is greater than
the centrifugal force (Fig. 17-33). Equilibrium between the horizontal
lift component and centrifugal force is reestablished either by decreasing
the bank, increasing the rate of turn, or a combination of the two changes.

A skidding turn results from an excess of centrifugal force
over the horizontal lift component, pulling the airplane toward the outside
of the turn. The rate of turn is too great for the angle of bank. Correction
of a skidding turn thus involves a reduction in the rate of turn, an increase
in bank, or a combination of the two changes.

To maintain a given rate of turn, the angle of bank must
be varied with the airspeed. This becomes particularly important in high
speed airplanes. For instance, at 400 miles per hour, an airplane must
be banked approximately 44 degrees to execute a standard rate turn (3 degrees
per second). At this angle of bank, only about 79 percent of the lift of
the airplane comprises the vertical component of the lift; the result is
a loss of altitude unless the angle of attack is increased sufficiently
to compensate for the loss of vertical lift.